Cardiothoracic ratios were estimated from 929 full-sized chest radiographs of representative samples of men and women of predominantly African origin aged 35–64 years living in rural and suburban communities in Jamaica. CTRs were greater in women than in men and increased with age; they were correlated with weight and with blood pressure levels. Mean CTRs were substantially greater in Jamaica than in a comparable sample of white persons living in Wales. These differences could not be explained by the prevalence of cardiovascular disorders including hypertension or by differences in weight or physical activity. An ethnic difference, possibly in the position of the heart within the chest or in the size of the thoracic diameter, is suggested. The greater CTR of the Negro should be taken into account when assessing heart size from chest films.